Induction device

ABSTRACT

A device for sustaining a plasma in a torch is provided. In certain examples, the device comprises a first electrode configured to couple to a power source and constructed and arranged to provide a loop current along a radial plane of the torch. In some examples, the radial plane of the torch is substantially perpendicular to a longitudinal axis of the torch.

PRIORITY APPLICATION

This application is a continuation of U.S. patent application Ser. No.11/218,912 filed on Sep. 2, 2005 which is a continuation-in-part of U.S.patent application Ser. No. 10/730,779 (now U.S. Pat. No. 7,106,438)filed on Dec. 9, 2003, which claimed priority to U.S. ProvisionalApplication No. 60/432,963 filed on Dec. 12, 2002, the entire disclosureof each of which is hereby incorporated herein by reference for allpurposes.

FIELD OF THE TECHNOLOGY

Certain examples relate to devices and methods for use in generating aplasma and to methods and apparatus for analyzing a sample introducedinto in a plasma generated by such devices.

BACKGROUND

Many inductively coupled plasma optical emission spectroscopy (ICP-OES)systems, inductively coupled plasma atomic absorption spectroscopy(ICP-AAS) systems, and inductively coupled plasma mass spectroscopy(ICP-MS) systems use a solenoid receptive of an RF electrical currentfor forming a plasma. However, the induced current generated by themagnetic field is skewed and non-homogeneous over the length of theinterior of the solenoid due to the helical configuration of thesolenoid. This non-homogeneity results in a variable temperaturedistribution within the plasma, which can affect sample excitation andthe trajectory of ions in the plasma. In addition, the solenoid is asingle element, which lacks flexibility in controlling the associatedinduced current formed by the magnetic field and the plasma/sampleexcitation.

SUMMARY

In accordance with a first aspect, a device for use in generating aplasma is provided. In certain examples, a device for generating aplasma in a torch having a longitudinal axis along which a flow of gasis introduced during operation of the torch and having a radial planesubstantially perpendicular to the longitudinal axis of the torch isdisclosed. In certain some, the device comprises a first electrodeconfigured to couple to a power source and constructed and arranged toprovide a loop current along the radial plane of the torch is provided.In certain examples, the device further includes a second electrodeconfigured to couple to a power source and constructed and arranged toprovide a loop current along the radial plane of the torch. In someexamples, each of the first and second electrodes comprises a platecomprising a symmetrical inner cross-section, e.g., a circular innercross-section. In certain examples, at least one spacer separates thefirst and second electrode. In other examples, the first electrode isconfigured to sustain a symmetrical plasma, or a substantiallysymmetrical plasma, in the torch, as described herein. In certainexamples, the first electrode, the second electrode or both may be inelectrical communication with a radio frequency source configured toprovide RF power to one or more of the electrodes. In some examples, thefirst electrode and the second electrode each have their own radiofrequency source. In certain examples, the first electrode, the secondelectrode or both, are in electrical communication with a groundingplate. The device may be configured for use in an inductively coupledplasma optical emission spectrometer, an inductively coupled plasmaatomic absorption spectrometer, an inductively coupled plasma massspectrometer or other suitable instrument.

In accordance with another aspect, a device for generating a plasma in atorch having a longitudinal axis along which a low of gas is introducedduring operation of the torch and having a radial plane substantiallyperpendicular to the longitudinal axis of the torch is disclosed. Incertain examples, the device comprises means for providing a loopcurrent along the radial plane of the torch. In some examples, the meansmay be an electrode or an equivalent structure that can provide a radiofrequency current along the radial plane of the torch. In certainexamples, the means may be a plate electrode, as described herein.

In accordance with an additional aspect, a method of sustaining a plasmain a torch having a longitudinal axis and having a radial planesubstantially perpendicular to the longitudinal axis of the torch isprovided. In certain examples, the method includes providing a gas flowalong the longitudinal axis of the torch, igniting the gas flow in thetorch, and providing a loop current along the radial plane to sustain aplasma in the torch. In some examples the method further includesconfiguring the plasma to be a substantially symmetrical plasma.

In accordance with another aspect, a substantially symmetrical plasma isdisclosed. In certain examples, the substantially symmetrical plasma maybe produced by igniting a gas flow in a torch and providing a loopcurrent along a radial plane substantially perpendicular to alongitudinal axis of the torch to sustain the substantially symmetricalplasma.

Additional aspects and examples will be recognized by the person ofordinary skill in the art, given the benefit of this disclosure, andcertain aspects and examples are described in more detail below.

BRIEF DESCRIPTION OF THE FIGURES

Certain examples are described below with reference to the accompanyingfigures in which:

FIG. 1 is schematic diagram of an inductively coupled plasma-opticalemission spectrometer (ICP-OES), in accordance with certain examples;

FIG. 2 is schematic diagram of an inductively coupled plasma-massspectrometer (ICP-MS), in accordance with certain examples;

FIG. 3 is a diagram of an ICP torch and a plasma, in accordance withcertain examples;

FIG. 4 is a side view of two electrodes, an ICP torch and a plasma, inaccordance with certain examples;

FIG. 5 is a front view of a first electrode for sustaining a plasma, theelectrode including an aperture, in accordance with certain examples;

FIG. 6 is a front view of a second electrode for sustaining a plasma,the electrode including an aperture, in accordance with certainexamples;

FIG. 7 is a side view of the electrode of FIG. 6, in accordance withcertain examples;

FIG. 8 is a perspective view of a unitary electrode, in accordance withcertain examples;

FIG. 9 is a front view of the electrode of FIG. 8, in accordance withcertain examples;

FIG. 10 is a side view of the electrode of FIG. 8, in accordance withcertain examples;

FIG. 11 is a top view of the electrode of FIG. 8, in accordance withcertain examples;

FIG. 12 is a perspective view of a magnetic field generated from a loopcurrent, in accordance with certain examples;

FIG. 13 is a diagram of an ICP torch showing the helical nature of asolenoid, in accordance with certain examples;

FIG. 14 is a diagram of a plurality of loop currents driven by a singleRF power source during alternating half cycles of a sinusoidallyalternating current, in accordance with certain examples;

FIGS. 15A and 15B show a torch and an induction device configured togenerate a current loop, in accordance with certain examples;

FIGS. 16A and 16B are induction devices, in accordance with certainexamples;

FIGS. 16C and 16D are induction devices, in accordance with certainexamples;

FIG. 17 is an axial view of an induction device, in accordance withcertain examples;

FIG. 18 is a radial view of the induction device of FIG. 17, inaccordance with certain examples;

FIG. 19 is a view of a plasma generated using the induction device ofFIGS. 17 and 18, viewed through a piece of welding glass, in accordancewith certain examples;

FIG. 20 is a view of a plasma generated using the induction device ofFIGS. 17 and 18, in accordance with certain examples;

FIG. 21 is an example of a symmetrical plasma generated using aninduction device with plate electrodes, in accordance with certainexamples;

FIG. 22 shows a radial view of induction devices with plate electrodesand a radial view of standard helical load coils, in accordance withcertain examples;

FIG. 23 shows an axial view of the induction devices with plateelectrodes of FIG. 22 and a radial view of standard helical load coilsof FIG. 22, in accordance with certain examples;

FIG. 24 is a 3-turn induction device, in accordance with certainexamples;

FIG. 25 is a graph showing the performance of a standard helical loadcoil for background and various metal species, in accordance withcertain examples;

FIG. 26 is a graph showing the performance of a 1S4T induction device (1spacer, 4 turns), normalized using the performance of the standardhelical load coil, for background and various metal species, inaccordance with certain examples;

FIG. 27 is a graph showing the performance of a 1-2S4T induction device(2 spacers, 4 turns), normalized using the performance of a standardhelical load coil, for background and various metal species, inaccordance with certain examples;

FIG. 28 is a graph comparing the performance of various inductiondevices for oxide and rhodium samples, in accordance with certainexamples;

FIG. 29 is graph comparing the rhodium signal from various inductiondevices, in accordance with certain examples;

FIG. 30 is a graph comparing the performance of various load coils for amagnesium sample, in accordance with certain examples; and

FIG. 31 is graph comparing the high mass performance of various loadcoils, in accordance with certain examples;

It will be recognized by the person of ordinary skill in the art, giventhe benefit of this disclosure, that the exemplary induction devices andother devices shown in the figures may not be to scale. Certain featuresor dimensions of the induction devices, the torches and the like mayhave been enlarged, reduced or distorted relative to other features tofacilitate a better understanding of aspects and examples disclosedherein.

DETAILED DESCRIPTION

Certain examples are described below to illustrate some of the manyapplications and uses of the induction device technology disclosedherein. These and other uses will be readily selected by the person ofordinary skill in the art, given the benefit of this disclosure. Unlessotherwise clear from the context, like numerals refer to similarstructures in different figures.

In accordance with certain examples, a device for generating asymmetrical or substantially symmetrical plasma is disclosed. As usedherein, “symmetrical plasma” refers to a plasma having a symmetricaltemperature profile, for a selected radial plane, extending radiallyfrom the center of the plasma. For example, a radial slice of a plasmawould have a life-saver shaped torus discharge associated with thatradial slice. For any given radius from the center of the torus, thetemperature is fairly uniform for any given angle of measurement aroundthe center for that radius. As used herein, “substantially symmetricalplasma” refers to a plasma that has a similar temperature profile, for aselected radial plane, extending radially from the center of the plasma,but the temperature profile may vary up to about 5% for any given radiusfrom the center of the torus discharge. Use of a symmetrical plasma, ora substantially symmetrical plasma, may provide significant benefitsincluding, but not limited to, less carbon build-up in the torch, lesstorch maintenance, an ion trajectory that is substantially parallel tothe axial direction, i.e., the longitudinal axis, of the torch, moreefficient sample transfer into the center of the plasma, and may allowfor reduced amounts of cooling gas or using no cooling gas at all. Alsoas used herein, “substantially perpendicular” refers to beingperpendicular to within about 5 degrees. It will be recognized by theperson of ordinary skill in the art, given the benefit of thisdisclosure, that a torch includes numerous radial planes perpendicularto the longitudinal axis of a torch, and that reference herein to a loopcurrent along a radial plane does not imply or suggest positioning ofthe loop current in any one specific position along the longitudinalaxis of the torch.

Referring now to FIG. 1, a schematic diagram of an exemplary inductivelycoupled plasma-optical emission spectrometer (ICP-OES) 100 is shown. Incertain examples, the ICP-OES 100 generally comprises a system fordirecting a carrier gas 102 to a torch 114 where the carrier gas 102 isionized to form a hot plasma 116 (e.g., 5,000-10,000K or greater). Insome examples, the plasma 116 comprises a preheating zone 190, aninduction zone 192, an initial radiation zone 194, an analytic zone 196and a plasma tail 198 (see FIG. 3). An atomized sample 104 may bedirected to the plasma 116 through a pump 106, nebulizer 108 and spraychamber 162. In the illustrative configuration shown in FIG. 1, a RFpower source 110 provides RF power to the plasma 116 by way of aninduction device 112. In plasma 116, excited sample atoms 104 may emitlight 134 as the excited atoms decay to a lower state. The emitted light134 may be collected by collection optics 118 and directed to aspectrometer 120 where it is spectrally resolved. A detector 122 may beoperative to detect the spectrally resolved light 134 and provide asignal 138, 140 to a microprocessor 122 and computer network 124 foranalysis. In examples where the species do not emit light, aninductively coupled atomic absorption spectrometer may be used toprovide light to the atomized species and a detector may be used todetect light absorption by the atomized species. Illustrative atomicabsorption spectrometers are available from PerkinElmer, Inc. andexemplary atomic absorption spectrometers are described, for example, incommonly owned U.S. Provisional No. 60/661,095 entitled “Plasmas andDevices Using Them” and filed on Mar. 11, 2005, the entire disclosure ofwhich is hereby incorporated herein by reference for all purposes.

In FIG. 1, the plasma 116 is shown as being viewed from a direction at aright angle to the longitudinal axis of the plasma 116, i.e., viewedradially or viewed along the radial axis. However, it will be understoodby the person of ordinary skill in the art, given the benefit of thisdisclosure, that the viewing of the plasma 116 may also be performedfrom a direction along the longitudinal axis 126 of the plasma 116,i.e., viewed axially. Detection of light emissions in the axialdirection can provide significant signal-to-noise benefits.

It will also be understood by the person or ordinary skill in the art,given the benefit of this disclosure, that the inductively coupledplasma may also be used with a mass spectrometer, (MS) 180 such as aquadrupole mass analyzer in an inductively coupled plasma-massspectrometer (ICP-MS) 100 as seen in FIG. 2. The RF power source 110operates generally in the range of about 1 to about 500 MHz,particularly 20-50 MHz, e.g., 27-40 MHz and powers of about 100 Watts toabout 10 kiloWatts are supplied to the electrodes to generate themagnetic field. Illustrative mass spectrometers are commerciallyavailable from PerkinElmer, Inc. and exemplary mass spectrometers aredescribed, for example, in commonly owned U.S. Provisional No.60/661,095 entitled “Plasmas and Devices Using Them” and filed on Mar.11, 2005.

FIG. 3 shows a more detailed schematic of the plasma 116 of FIGS. 1 and2. The torch 114 includes three concentric tubes 114, 150, and 148. Theinnermost tube 148, provides atomized flow 146 of the sample into theplasma 116. The middle tube 150, provides auxiliary gas flow 144 to theplasma 116. The outermost tube 114, provides carrier gas flow 128 forsustaining the plasma. The carrier gas flow 128 may be directed to theplasma 116 in a laminar flow about the middle tube 150. The auxiliarygas flow 144 may be directed to the plasma 116 within the middle tube150 and the atomized sample flow 146 may be directed to the plasma 116from the spray chamber 162 along the innermost tube 148. The RF current130, 132 in the load coil 112 may form a magnetic field within the loadcoil 112 so as to confine the plasma 116 therein.

The plasmas shown in FIGS. 1-3, and shown in certain other figuresdescribed herein, can be generated using numerous different electrodeconfigurations. FIGS. 4-11 show various configurations of an electrode152, 156, 158. In FIG. 4, the electrode 152 comprises two substantiallyparallel plates 152 a, 152 b positioned at a distance ‘L’ from oneanother. In certain examples, the substantially parallel plates have awidth of about 20 mm to about 200 mm, e.g., about 40 mm, and a length ofabout 30 mm to about 90 mm, e.g., about 70 mm. Each of the parallelplates 152 a, 152 b includes an aperture 154 through which the torch 114may be positioned such that the torch 114, the innermost tube 148, themiddle tube 150 and the aperture 154 are aligned along an axis 126. Theexact dimensions and shapes of the aperture may vary and may be anysuitable dimensions and shapes that can accept a torch. For example, theaperture may be generally circular and have a diameter of about 10 mm toabout 60 mm, may be square or rectangular shaped and have dimensions ofabout 20 mm to about 60 mm wide by about 20 mm to about 100 mm long, maybe triangular, oval, ovoid, or other suitable geometries. If a smalldiameter torch is used such as a “low flow” torch then the diameter ofthe aperture can be reduced proportionally to accommodate the torch. Incertain examples, the aperture may be sized such that it is about 0-50%or typically about 3% larger than the torch, whereas in other examples,the torch may contact the plates, e.g., some portion of the torch maycontact a surface of a plate, without any substantial operationalproblems. The substantially parallel plates 152 a, 152 b have athickness of ‘t.’ In some examples, each of plates 152 a and 152 b havethe same thickness, whereas in other examples plates 152 a and 152 b mayhave different thicknesses. In certain examples, the thickness of theplates is from about 0.025 mm (e.g., such as a metallized plating on aninsulator, an example of this would be copper, nickel, silver, or goldplating on a ceramic substrate) to about 20 mm, more particularly, about0.5 mm to about 5 mm, or any particular thickness within these exemplaryranges. The aperture 154 of the electrode 152 may also include a slot164, of width ‘w’ such that the aperture 154 is in communication withits surroundings. The width of the slot may vary from about 0.5 mm toabout 20 mm, more particularly, about 1 mm, to about 3 mm, e.g., about 1mm to about 2 mm.

In accordance with certain examples, the electrodes may be constructedfrom the same or different materials. In certain examples, theelectrodes may be constructed from conductive materials such as, forexample, aluminum, gold, copper, brass, steel, stainless steel,conductive ceramics and mixtures and alloys thereof. In other examplesthe electrodes may be constructed from non-conductive materials thatinclude a plating or coating of one or more conductive materials. Insome examples, the electrodes may be constructed from materials capableof withstanding high temperatures and resisting melting when exposed tothe high circulating currents required to generate the plasma. These andother suitable materials for constructing the electrodes will be readilyselected by the person of ordinary skill in the art, given the benefitof this disclosure.

Referring to FIGS. 4 and 5, the electrode 152 may be generally comprisedof a square or rectangular planar shape, though it may be a wire as seenin FIG. 12. In certain examples, the RF current supplied to the planarelectrode creates a planar loop current 172 a, which generates atoroidal magnetic field 182 through the aperture 154 (see FIG. 12). Theplanar current loop may be substantially parallel to a radial plane,which is substantially perpendicular to the longitudinal axis of thetorch. The toroidal magnetic field may be operative to generate andsustain a plasma within a torch, such as torch 114 shown in FIG. 3. In atypical plasma, argon gas may be introduced into the torch at flow ratesof about 15-20 Liters per minute. A plasma may be generated using aspark or an arc to ignite the argon gas. The toroidal magnetic fieldcauses argon atoms and ions to collide, which results in a superheatedenvironment, e.g., about 5,000-10,000 K or higher, that forms theplasma.

Referring now to FIGS. 6 and 7, the electrode 156 may be of a roundednature having an outside diameter of D₁ and inside aperture diameter ofD₂. In some examples, the outside diameter ranges from about 10 mm toabout 20 cm, more particularly about 25 mm to about 10 cm, e.g., about30 mm to about 50 mm, and the inside diameter ranges from about 10 mm toabout 15 cm, more particularly, from about 5 mm to about 5 cm, e.g.,about 20 mm to about 24 mm. In certain examples, electrodes 152, 156 ofFIGS. 4-7 may be distinct elements which are supplied independently withRF electrical current 172 and typically of opposite polarity (thoughopposite polarity is not required for operation). In other examples,electrodes 152, 156 of FIGS. 4-7 may be elements in electricalcommunication and may each be suitably designed to provide the desiredpolarity to generate a magnetic field.

In accordance with certain examples, one part 176 of the electrode 152may be supplied with the RF power while a second part 178 of theelectrode 152 may be tied to a ground 174. In some examples, theelectrode may be grounded to the instrument chassis, whereas in otherexamples, the electrode may be mounted and grounded to a groundingplate, which itself may be grounded in a suitable manner. During arcignition of the plasma 116, if the ignition arc makes contact withelectrode 152, any unwanted electric currents set up in the electrode152 may be directed to the ground point 174 and not through to the RFpower supply 110. The RF power and frequency supplied to each electrode152 may be independently controlled and varied for optimum performance.For instance, each electrode 152 may be operated at a differentfrequency in order to optimize the plasma emission and excitation. Inaddition, one electrode (or both electrodes) may be operated in acontinuous power mode while the other electrode can be modulated (e.g.,pulsed or gated). In certain examples, the distance, ‘L’, between theelectrodes 152 may be adjusted since the electrodes 152 are notconnected to one another, which can result in adjustment of the powerdistribution within the plasma 116. Yet further, the diameter, D₂ of theaperture 154 may be independently adjusted in order to adjust thecoupling characteristics between the RF power supply 110 and the plasma116.

In accordance with certain examples, spacers may be placed between someportion of the electrodes to control the distance between theelectrodes. In certain examples, the spacers are constructed using thesame materials used to construct the electrodes. In some examples, thespacers are made from a material having substantially the samecoefficient of thermal expansion as the electrode material so that asthe electrode expands and contracts with different temperatures, thespacer expands and contracts at about the same rate. In some examples,the spacers are stainless steel washers, brash washers, copper washersor washers made from other suitable conductive materials. In certainexamples, the spacers are washers that are sized suitably to receive abolt or nut that connects the electrodes. By using one or more spacers,the distance between the electrodes may be easily reproduced and/oraltered. It will be within the ability of the person of ordinary skillin the art, given the benefit of this disclosure, to select suitablematerials and shapes for spacers for use with the electrodes disclosedherein.

Referring now to FIGS. 8-11, induction device 158 is shown as includingtwo electrodes 166, 168 connected to a common electrical ground 170.Induction coil 158 may be configured as a helical coil with electrodes166 and 168 being in electrical communication with each other. When RFcurrent 172 is supplied to induction device 158, loop currents 172 a aregenerated, which creates a toroidal magnetic field. Loop currents 172 aare substantially parallel to the planar surfaces of electrodes 166 and168 and would be substantially perpendicular to the longitudinal axis ofthe torch. Induction coil 158 may be grounded at common electricalground 170 (see FIG. 10) to prevent unwanted arcing, which can result inmelting of electrodes 166 and 168. In certain examples, electrodes 166and 168 are spaced a distance L from each other (see FIGS. 8 and 10).The exact distance between electrodes 166 and 168 can vary and exemplarydistances include, but are not limited to, about 1 mm to about 5 cm,more particularly about 2 mm to about 2 cm, e.g., about 5 mm to about 15mm. In certain examples, electrodes 166 and 168 are arrangedsubstantially perpendicular to a mounting surface. In other examples,electrodes 166 and 168 may be tilted at an angle so that the axialdimension of the torch and the radial dimension of the electrodes aresubstantially perpendicular. In some examples, each of electrodes 166and 168 may be angled in the same direction, whereas in other examples,electrodes 166 and 168 may be angled in opposite directions. The personof ordinary skill in the art, given the benefit of this disclosure, willbe able to select suitable configurations and angles for the electrodesof the illustrative induction devices disclosed herein.

In accordance with certain examples, an exemplary configuration of aninduction device surrounding a torch is shown in FIG. 13. Inductiondevice 112 may surround concentric fluid conduits 114, 150 and 148.Carrier gas flow 128 may be introduced into the torch to provide gas forgeneration of the plasma using induction device 112. Auxiliary gas flow144 may be introduced into concentric tube 150 to provide gas forcontrolling the plasma position relative to the injector 148. Sampleflow 146 may enter aerosol conduit 148 where it is sprayed into theplasma generated by induction device 112. The exact flow rates of thevarious gas species may vary. For example, the carrier gas is typicallyintroduced at a flow rate of about 10 L/min to about 20 L/min, e.g.,about 15-16 L/minute. The auxiliary gas is typically introduced at aflow rate of about 0 L/min to about 2 L/minute. The sample can beintroduced at a suitable flow rate to provide desolvation and/oratomization of the sample. In some examples, the sample is introduced ata flow rate of about 0.1 L/minute to about 2 L/minute. Additional flowrates for the carrier gas, auxiliary gas and sample will be readilyselected by the person of ordinary skill in the art, given the benefitof this disclosure.

Referring now to FIG. 14, a plurality of loop currents 184 a, 184 b areshown generated from a single RF electric current source 110. Forclarity of illustration, the electrodes have been omitted from FIG. 14.The loop currents 184 a, 184 b are generated by applying a current ofopposite polarities to apposing electrodes. The loop currents 184 a, 184b may be oriented with respect to one another in a suitable manner suchthat the alternating electric current 172 a in a first loop current 184a flows in the same direction as that of the alternating electriccurrent 172 b in a second loop current 184 b during alternating halfcycles of a sinusoidally alternating current. This configuration allowsfor the plurality of loop currents 184 a, 184 b to be driven from asingle power source 110 so as to generate magnetic fields 182 a, 182 bhaving the same spatial orientation. An example of this can be seen inFIGS. 17 and 18, where diagonally apposing legs of each coil 1002 and1004 are driven from a single RF source located directly below, and theremaining two legs, also diagonally apposing, are commonly connected toa grounded plate 1006. The plane of the loop currents 184 a, 184 b isalso substantially perpendicular to the longitudinal axis 126 of thetorch and is substantially parallel to a radial plane of the torch. Incertain examples, the device includes two or more electrodes constructedand arranged to generate a magnetic field to sustain a symmetrical orsubstantially symmetrical plasma. Certain exemplary electrodes arediscussed above in reference to FIGS. 1-14 and other exemplaryelectrodes are discussed below.

In accordance with certain examples, a device for generating a plasmacomprising a first electrode constructed and arranged to provide a firstloop current along a radial plane that is substantially perpendicular toa longitudinal axis of a torch is disclosed. Referring to FIGS. 15A and15B, device 400 includes electrode 402, which has a slot 404 and anaperture 406 for receiving a torch 410. The electrode 402 has a circularinner cross-section that is substantially symmetrical. In certainexamples, the diameter of the inner cross-section is about 10 mm toabout 60 mm, more particularly about 20 mm to about 30 mm, e.g., about20 mm to about 23 mm. In some examples, the diameter of the innercross-section is selected such that about 1 mm of distance separates theouter surface of the torch 410 from the inner portion of the electrode402. The electrode 402 may be positioned such that it is substantiallyperpendicular to the longitudinal axis (shown in FIG. 15B as a dottedline) of the torch 410. The slot 404 of the electrode 402 may beconfigured such that the current provided to electrode 402 will take theform of a loop, such as a loop current 412 shown in FIG. 15B. In someexamples, the loop current 412 is substantially perpendicular to thelongitudinal axis of the torch 410, e.g., the plane of the loop currentis substantially perpendicular to the longitudinal axis of the torch410. Use of a substantially perpendicular loop current may generateand/or sustain a plasma that has a more symmetrical temperaturedistribution, for a selected radial plane, than plasmas generated usinghelical load coils. In certain examples, a symmetrical plasma, orsubstantially symmetrical plasma, is sustained using an electrode, suchas the electrode 402, positioned substantially perpendicular to thelongitudinal axis of the torch 410. In certain examples, the selectedoverall shape of the electrode may vary. For example and as shown inFIG. 15A, electrode 402 is configured with an overall rectangular shape.However, other suitable shapes, such as circles, triangles, rings,ellipses, toroids and the like may also be used. The first electrode maybe mounted to a grounding plate as described herein.

In certain examples, a second electrode similar to the electrode 402 inFIG. 15A may also be constructed and arranged parallel to a radialplane, which is substantially perpendicular to a longitudinal axis of atorch 410. In other examples, the plane of the second loop current maybe substantially parallel to the plane of the first loop current. Insome examples, the first and second loop currents may flow in the samedirection, whereas in other examples the first and second loop currentsmay flow in an opposite direction. In examples where more than oneelectrode is used, a single RF source, such as RF source 420 shown inFIG. 15A, may provide RF power to each of the first and secondelectrodes, or separate RF sources may provide RF power to the first andsecond electrodes. In some examples, spacers are used to separate thefirst and second electrodes. In examples where a single RF source isused to provide RF power to the first and second electrodes and wherespacers are used, the spacers may be made of a conductive material,e.g., copper, brass, gold and the like. In examples where separate RFsources are used to provide RF power to the first and second electrodesand where spacers are used, the spacers may be made of a non-conductivematerial, e.g., glass, plastics, etc., to prevent current flow from thefirst electrode to the second electrode.

In accordance with certain examples, the first electrode, the secondelectrode or both may be grounded to a grounding plate. For example, andreferring now to FIGS. 16A and 16B, induction device 500 may include afirst electrode 502 and a second electrode 504 each mounted to agrounding plate 506. In the example shown in FIGS. 16A and 16B, theelectrodes 502 and 504 may be mounted to the grounding plate 506 usingsupports 503 and 505, respectively. In certain examples, the diagonallyapposing legs of each electrode 502 and 504 may be driven from a singleRF source located directly below, the remaining two legs, alsodiagonally apposing, may be commonly connected to a grounding plate 506,and all components may be electrically connected through the fouridentical mounts typically identified as 503 and 505. The supports 503and 505 may provide electrical communication between the electrodes 502and 504 and the grounding plate 506 such that during arc ignition of theplasma, if an ignition arc makes contact with the electrodes 502, 504,any unwanted electric currents set up in the electrodes 502, 504 may bedirected to the grounding plate 506 and not passed through to the RFpower supply (not shown) in electrical communication with the electrodes502 and 504. Use of the electrodes 502 and 504 with the grounding plate506 may provide a more symmetrical plasma, which can improve detectionlimits of certain species (as discussed in more detail in the examplesherein), than plasmas generated using helical load coils. For example,using existing helical load coils there may exist areas of the plasmathat have a reduced temperature and are inefficient at desolvation andatomization due to the plasmas tendency to follow the helix of the loadcoil resulting in a non-uniform plasma discharge. Using examples of theinduction devices disclosed herein, a plasma having a more symmetricaltemperature distribution, for a selected radial plane, is generatedwhich can provide for more even desolvation and atomization, whichresults in improved performance, extended torch life, and less carbonbuildup when used with organics.

In certain examples, an induction device as disclosed herein may beoperated at much lower powers than conventional helical load coils. Forexample, a power of about 800 Watts to about 1250 Watts, e.g., about 900Watts to about 1050 Watts, may be used with an induction devicedisclosed herein to sustain a plasma suitable for use, for example, ininstruments for chemical analysis. For comparative purposes only, atypical conventional helical load coil uses about 1450 Watts of power ormore to sustain a plasma suitable for chemical analysis. In someexamples, an induction device provided herein is configured to use about10-20% less power than a helical load coil.

In accordance with certain examples, the exact thickness of theelectrode and the grounding plate can vary depending, for example, onthe intended use of the device, the desired shape of the plasma, etc. Incertain examples, the electrode is about 0.05-10 mm thick, moreparticularly, about 1-7 mm, thick, e.g., about 1, 2, 3, 4, 5, or 6 mmthick or any dimensions between these illustrative thicknesses.Similarly, the exact dimensions and thickness of the grounding plate mayvary. For example, the grounding plate may be from about 5 mm to about500 mm wide to about 5 mm to about 500 mm long, or it could be as largeas the whole instrument chassis itself, and may have a thickness fromabout 0.025 mm thick to about 20 mm thick. It will be within the abilityof the person of ordinary skill in the art, given the benefit of thisdisclosure, to select suitable electrode and grounding plate dimensionsand thicknesses to provide a desired plasma shape.

In accordance with certain examples, each electrode of an inductiondevice may be individually tuned or controlled. Referring to FIG. 16C,an induction device 600 includes electrodes 602 and 604 in electricalcommunication with a grounding plate 606 through supports 603 and 605,respectively. The grounding plate 606 may be configured to preventunwanted arcing, which can result in melting of the electrodes 602, 604.In certain configurations, the grounding plate 606 may itself begrounded to the instrument chassis. An RF source 610 may be configuredto provide a current to the electrode 602, and an RF source 620 may beconfigured to provide a current to the electrode 604. The currentsupplied to the electrodes 602 and 604 may be the same or may bedifferent. The current may also be altered or changed during operationof the plasma to change the shape and/or temperature of the plasma. Inother examples, a single RF source may be configured to provide currentto both electrodes 602, 604. For example and referring to FIG. 16D, aninduction device 650 includes electrodes 602 and 604 in electricalcommunication with a grounding plate 606 through supports 603 and 605,respectively. An RF source 660 may be configured to provide a current toeach of the electrodes 602 and 604. Even though a single RF source maybe used to provide current to the electrodes 602 and 604, the currentsupplied to each electrode may or may not be the same. For example,suitable electronic circuitry may be implemented to supply one of theelectrodes with a different current. The person of ordinary skill in theart, given the benefit of this disclosure, will be able to designsuitable induction devices using one or more RF sources.

In accordance with certain examples, a device for sustaining a plasma ina torch having a longitudinal axis along which a flow of gas isintroduced during operation of the torch and having a radial planesubstantially perpendicular to the longitudinal axis of the torch isprovided. In certain examples, the device includes means for providing aloop current along a radial plane of the torch. Suitable means include,but are not limited to, any one or more of the electrodes disclosedherein or other suitable devices that can provide loop currents along aradial plane.

In accordance with certain examples, a method of sustaining a plasma ina torch having a longitudinal axis and having a radial planesubstantially perpendicular to the longitudinal axis of the torch isdisclosed. In certain examples, the method includes providing a gas flowalong the longitudinal axis of the torch, igniting the gas flow in thetorch, and providing a loop current along the radial plane to sustain aplasma in the torch. The loop current may be provided using any one ormore of the electrodes disclosed herein or other suitable electrodeconfigurations that may provide a loop current along a radial plane. Incertain examples, the plasma which is sustained using the methoddescribed herein is a substantially symmetrical plasma.

In accordance with certain examples, a signal from the plasma may bemonitored between the two or more of the electrodes of the inductiondevice. In some examples, radial detection of optical emission ofexcited species between the electrodes, or above the electrodes, may beperformed using standard optical detectors. In other examples, axialdetection may be used to monitor the signal from the plasma or speciesin the plasma.

Suitable electronic components for providing current to the electrodeswill be readily selected by the person of ordinary skill in the art,given the benefit of this disclosure. For example, illustrative RFsources and oscillators may be found in U.S. Pat. No. 6,329,757, theentire disclosure of which is hereby incorporated herein by referencefor all purposes.

Certain specific examples are discussed in more detail below to furtherillustrate aspects and examples of the technology.

Example 1—Plate Induction Coil

An induction device 1000 was assembled with two electrodes 1002 and1004, each of which were grounded to grounding plate 1006 (see FIGS. 17and 18). The electrodes 1002 and 1004 were each 2 mm thick platesmachined out of 50-52 sheet aluminum. A modified face plate wasinstalled and evaluated on Optima 2000 and Optima 4000 instruments,available from PerkinElmer, Inc. This face plate, as shown in FIGS. 17and 18, included the replacement of the helical load coil with thegrounding plate 1006, inductors 1002 and 1004 and mounts. Very minormodifications were needed to the faceplate to include clearance holeswhere needed for the bolts securing mounting blocks. No functionalchanges were made to the generator. The modified instrument was testedto ensure that it met all of the instrument specifications at a flowrate of about 15 L/minute (see plasma shown in FIG. 19 (viewed throughwelding glass) and FIG. 20). The temperature of the oscillator heatsink, for the same input power, was cooler than when the helical loadcoil was used. A lower oscillator heat sink temperature indicated morepower was getting to the plasma. The plasma could also be maintained ata lower power (about 1250 Watts) than the power used with a conventionalhelical load coil (about 1450 Watts). In addition, when the instrumentwas operated at 1450 Watts, the induction device was better able tohandle the sample loading than the helical coil at the same powersetting.

Example 2—Symmetrical Plasma Discharge

An induction device was constructed to generate a substantiallysymmetrical plasma discharge, as shown in FIG. 21. Referring to FIGS. 22and 23, induction devices 1102 (three plates) and 1104 (two plates) wereeach used to generate a substantially symmetrical plasma discharge andconventional helical load coils 1106 (three turns) and 1108 (two turns)are shown for comparison purposes. The blocks that are used to mount thehelical load coil to the oscillator also include existing hardware(screws) that hold the block to the faceplate. These screws were used toconnect the induction device legs to the blocks after the helical coilwas removed. No additional modifications were needed. With aconventional helical load coil, the discharge currents and resultingplasma temperatures tend to follow the helix of the load coil, whichresults in a non-uniform plasma discharge. This non-uniform dischargehas many disadvantages including skewed ion trajectory, non-uniformheating of the sample, sample spilling around the outside of the plasmadue to the non-flat bottom of the plasma, and less well defined regionsof ionization. By using the induction devices disclosed herein insteadof the wound copper tubing in helical load coils, it is possible tobetter control the temperature gradient in the plasma and provide a moresymmetrical plasma. Additional tuning can be provided using theinduction device disclosed herein by changing the spacing between platesof the induction device. For example it may be beneficial to stagger thespacing in between the plates of an induction device. If changing thespacing is attempted with a conventional coil by increasing the spacingbetween the load coil turns the plasma tends to become even moreasymmetrical.

It was found that use of the induction device disclosed herein improvedsensitivities, especially in the low mass range (5-60 Atomic Mass Units(AMU)), lowered oxide ratios, and lowered working pressures. Asymmetrical plasma, for example, also allows for running volatilesamples without having the sample escape around the side of the plasma,provides a better defined ionization region, and removes the highbackground spike where the load coil peals off at the top of the plasmaplume. FIG. 24 shows an exemplary plate induction device 700 that wastested in an Elan 6000 ICP mass spectrometer, available fromPerkinElmer, Inc. Induction device 700 worked with both tube and solidstate RF generators, and on both a ICP mass spectrometer and ICP-OESgenerators.

Example 3—Spacer Combinations

Various induction devices having different numbers of turns anddifferent spacers were tested using ICP mass spectroscopy. The standardElan generator uses a 3 turn load coil made out of ⅛″ copper tubing andelectrically connected to the generator using Swaglock fittings. Whenthe induction devices were used, they were directly bolted to theexisting electrodes in place of the Swaglock fittings. For eachinduction device, the unit (an ELAN 6000 commercially available fromPerkinElmer, Inc.) was optimized and then data was gathered fordifferent aspirated species including magnesium (Mg), rhodium (Rh), lead(Pb), cerium (Ce), cerium oxide (CeO), barium (Ba), barium+2 (Ba++), andbackground signal (BG 220). The data was normalized to the maximumsensitivity signal and plotted with the results shown in FIGS. 25-31.The various sensitivities were normalized to the maximum signal detectedusing a standard ELAN 6000 having a helical load coil (⅛ inch diametercopper tubing with 3 turns). The combination of induction devices thatwere tested were mixes of Standard 5 turn L2 inductor devices, a 4 turnL2 inductor and the 0.875 diameter plate induction device with differentnumber of spacers between the plates. The term “L2 inductor,” which isused in the abbreviations herein, represents part of the internalimpedance matching coil, located inside the RF generator. The normaloperating power (unless otherwise specified) was 1000 watts. Each spacerwas a 632 brass washer. The combinations of induction devices that weretested are listed below. The abbreviations are referenced in FIGS.25-31.

-   -   1. Standard Load coil with Standard 5 turn L2 inductor.    -   2. 1S5T—Plate induction device, one spacer between plates and        standard 5 turn L2 inductor.    -   3. 1S4T—Plate induction device, one spacer between plates and 4        turn L2 inductor.    -   4. 1-2S4T—Plate induction device, one rear spacer, 2 front        spacers and 4 turn L2 inductor.    -   5. 1-2S5T—Plate induction device, one rear spacer, 2 front        spacers and 5 turn    -   Standard L2 inductor.    -   6. 2S5T—Plate induction device, two spacers between plates and 5        turn Standard L2 inductor.    -   7. 2S4T—Plate induction device, two spacers between plates and 4        turn L2 inductor.    -   8. 3S4T—Plate induction device, three spacers between plates and        4 turn L2 inductor.    -   9. 3S5T—Plate induction device, three spacers between plates and        5 turn Standard L2 inductor.        When the data was plotted, data measured with the Plate        induction device and Standard 5 turn L2 inductor had a double        mountain indicating a pinch was present. The pinch refers to a        secondary discharge between the plasma plume and the sampling        interface. The pinch discharge can be eliminated by minimizing        the plasma potential at the interface cone.

FIG. 25 is plot for each of the detected samples using a Standard ELAN6000 mass spectrometer to which the all measurements were compared. Theleft axis represents normalized intensity, the x-axis representnebulizer flow in L/min and the right axis represents eithercounts/second (for the BG 220 measurements) or percentage of oxides (forthe CeO/Ce and Ba++/Ba measurements). The maximum sensitivity peaks ofthe different elements occur at different nebulizer flow rates. Forsensitivity reasons, it is preferable for the maximum sensitivity of theelements to occur at a lower flow rate than the flow rate used toobserve the maximum sensitivity of oxides, such as cerium oxide, asoxides tend to interfere with measurement of other species.

Referring now to FIG. 26, the results from testing a 4 turn L2 internalimpedance matching inductor and configuration (1S4T) are shown. Usingthe 1S4T device, the maximum sensitivity peak of the different elements(Mg, Rh, Pb) occurred at the same nebulizer flow (about 0.84). Thesingle spacer gave highest mid mass sensitivity (mid mass typicallyrefers to species having atomic mass units between 60-180 AMU, and highmass refers to species having atomic mass units from 180-238 AMU).

Referring now to FIG. 27, the results from testing a double front singlerear spacer 4 turn L2 inductor (1-2S4T) are shown. Using the 1-2S4Tdevice it was possible to separate the magnesium, rhodium, and leadsignals away from the top of oxide mountain (TOM) observed with Ce/CeO.

Referring now to FIG. 28, various load coils were tested for rhodiumsensitivity as compared to cerium oxide/cerium signal. Tested devicesincluded the standard helical load coil and a plate induction devicehaving one rear spacer, 2 front spacers and 4 turn L2 inductor (1-2S4T).Using the 1-2S4T device, the maximum sensitivity peak of the rhodium wasshifted to lower flow rates and away from the maximum sensitivity peakof the cerium oxide/cerium, indicating the 1-2S4T device provided betterrhodium sensitivity than the standard coil.

Referring now to FIG. 29, the rhodium signal was normalized to themaximum standard signal from the ELAN 6000. Use of the 1S4T deviceprovided a 30% signal increase as compared to the standard helical loadcoil. Use of the 1-2S4T device separated the oxide mountain observedwith the standard coil.

Referring now to FIG. 30, magnesium was used to measure the performanceof the various devices. The 1-2S4T device exhibited the best low massperformance of all the devices tested.

Referring now to FIG. 31, lead was used to measure the high massperformance of the various devices. The plate induction devices all gavepoorer high mass performance than the standard ELAN 6000 at nebulizerflow rates greater than about 0.96. At lower nebulizer flow rates,however, the plate induction devices all give better high massperformance than the standard ELAN 6000, which may allow detection ofhigh mass species using reduced amounts of sample.

Example 4—Optical Emission Detection Limits Using Plate Induction Coils

An optical emission spectrometer (Optima 3000 obtained from PerkinElmer,Inc.) was fitted with either a helical load coil or a plate inductiondevice to measure the detection limits for arsenic (As), cadmium (Cd),chromium (Cr), manganese (Mn), lead (Pb) and selenium (Se). The helicalload coil was the standard 3/16″ diameter copper coil. The plateinduction device included two circular electrodes each having anaperture for receiving a torch. For comparison purposes only, thedetection limits using the helical load coil and the plate inductiondevice are shown in Table I below.

TABLE I Detection Limit Detection Limit (ppb) Emission (ppb) PlateInduction Element Wavelength (nm) Helical Load Coil Device Arsenic 19734 10 Cadmium 214 0.8 0.3 Chromium 205 1.7 0.6 Manganese 257 0.15 0.1Lead 220 8 4 Selenium 196 26 14While detection limits on newer instruments may be better than thoselisted in Table I above, a relative comparison of the detection limitsreveals that the detection limits using the plate induction device wereconsistently lower than the detection limits obtained using the helicalload coil.

When introducing elements of the examples disclosed herein, the articles“a,” “an,” “the” and “said” are intended to mean that there are one ormore of the elements. The terms “comprising,” “including” and “having”are intended to be open ended and mean that there may be additionalelements other than the listed elements. It will be recognized by theperson of ordinary skill in the art, given the benefit of thisdisclosure, that various components of the examples can be interchangedor substituted with various components in other examples. Should themeaning of the terms of any of the patents, patent applications orpublications incorporated herein by reference conflict with the meaningof the terms used in this disclosure, the meaning of the terms in thisdisclosure are intended to be controlling.

Although certain aspects, examples and embodiments have been describedabove, it will be recognized by the person of ordinary skill in the art,given the benefit of this disclosure, that additions, substitutions,modifications, and alterations of the disclosed illustrative aspects,examples and embodiments are possible.

What is claimed is:
 1. A method of detecting an analyte speciescomprising: introducing an analyte species into a torch having alongitudinal axis along which a flow of gas is introduced duringoperation of the torch and having a radial plane substantiallyperpendicular to the longitudinal axis of the torch; providing a loopcurrent along the radial plane of the torch using a device configured togenerate an inductively coupled plasma in the torch, the devicecomprising a first plate electrode configured to couple to a powersource; and detecting an optical emission from the analyte speciesintroduced into the plasma generated in the torch.
 2. The method ofclaim 1, further comprising configuring the device to comprise a secondplate electrode configured to couple to a power source, wherein thesecond plate electrode is constructed and arranged to provide a loopcurrent along the radial plane of the torch.
 3. The method of claim 2,further comprising configuring each of the first and second plateelectrodes to comprise a symmetrical inner cross-section.
 4. The methodof claim 3, further comprising configuring the symmetrical innercross-section to be circular.
 5. The method of claim 2, furthercomprising configuring the device with at least one spacer separatingthe first plate electrode and the second plate electrode.
 6. The methodof claim 2, further comprising configuring the first plate electrode tosustain a substantially symmetrical plasma in the torch.
 7. The methodof claim 1, further comprising configuring the device to comprise aradio frequency source in electrical communication with the first plateelectrode.
 8. The method of claim 7, further comprising configuring theradio frequency source to provide radio frequencies of about 1 MHz toabout 1000 MHz at a power of about 10 Watts to about 10,000 Watts. 9.The method of claim 7, further comprising configuring the device with asecond radio frequency source in electrical communication with thesecond plate electrode.
 10. The method of claim 2, further comprisingconfiguring the device to comprise a single radio frequency source inelectrical communication with the first plate electrode and the secondplate electrode.
 11. The method of claim 10, further comprisingconfiguring the single radio frequency source to provide radio frequencyenergy of about 1 MHz to about 1,000 MHz at a power of about 10 Watts toabout 10,000 Watts.
 12. The method of claim 2, further comprisingconfiguring the device with a grounding plate in electricalcommunication with the first plate electrode and the second plateelectrode.
 13. The method of claim 1, further comprising axiallydetecting the optical emission from the analyte species.
 14. The methodof claim 1, further comprising using collection optics to provide theoptical emission from the analyte species to a detector.
 15. The methodof claim 14, further comprising configuring the detector to spectrallyresolve the optical emission from the analyte species.
 16. The method ofclaim 1, further comprising radially detecting the optical emission fromthe analyte species.
 17. The method of claim 1, further comprisingintroducing the analyte species from a sample introduction devicefluidically coupled to the torch.